Injection molding apparatus

ABSTRACT

A method of pressure die-casting a member by providing a mandrel in the die cavity of a mould to exclude air therefrom, and withdrawing the mandrel at a controlled rate as molten metal is delivered to the mould so that the incoming metal occupies the die cavity as it is vacated by the mandrel and thus air is not trapped in the mould or entrained in the incoming molten metal.

I United States Patent 1 1111 3,759,315 'Smart 1 Sept. 18, 1973 [54] INJECTION MOLDING APPARATUS 2,137,764 11/1938 Wagner 164/313 3,387,646 6/1968 Lauth 164/120 [75] lnvenmr- Malvem' 2,398,318 4/1946 MacMillin et al. 164/113 Austral"! 3,270,377 9/1966 Parker 164/313 x [73] Assignee: Reginald Richard Mills, Burwood, FOREIGN PATENTS OR APPLICATIONS Victoria, Australia; a part interest 3 475,544 11/1937 Great Britain 164/313 [22] Filed: Mar. 1, 1971 [21] Appl. No.: 119,578 Primary Examiner-J. Spencer Overholser Assistant ExaminerDavid S. Safran [30] Foreign Application Priority Data Att0mey wendemth Lmd & Ponack Mar, 9, 1970 Australia 565/70 [57] ABSTRACT 52 US. Cl ..164 120 164113,164 312, l 1 I 4250242 A method of pressure dle-castmg a member by provid- 51 161.121 32211 17/00 mandrel the die cavity a exclude [58] Field of Search 164/113 120 303 and withdrawing the mandrel at a 164/313 312 319 27 trolled rate as molten metal is delivered to the mould so that the incoming metal occupies the die cavity as it 56] References Cited is vacated by the mandrel and thus air is not trapped in UNITED STATES PATENTS the mould or entrained in the incoming molten metal.

6 1959 Miiller et a1. 164/120 4 Claims, 1 Drawing Figure Pa ten-ted Sept. 18, 1973 3, 759,315

HORACE STANLEY SMART, Inventor By, J

Attorneys 1 INJECTION MOLDING APPARATUS BACKGROUND OF THE INVENTION This invention relates to a method for the production of pressure die-cast members such as rods and bars of tubular and solid sections and more particularly to a method for the production by pressure die-casting of rods and bars of tubular and solid sections from zinc based alloys.

Whereas metal rods and bars of tubular and solid section are normally manufactured by the processes of rolling, drawing, extrusion and gravity casting, (whether as individual or combined processes), some metals have not been available of acceptable standards for industry in these forms as known methods of production are not physically and/or commercially practical. Specifically, zinc based die-casting alloys are particular examples.

Zinc based alloys have many physical properties, such as tensile strength, rust resistance, plate-ability, and the like, which would render them a commercial substitute in many applications for other metals and particularly non-ferrous metals if they were also available in rod, tube and bar form of acceptable quality standards. Such a substitution would currently represent a substantial cost saving because of the low cost of zinc based alloys in comparison to the copper based alloys. For example, substitution of zinc alloy rods for brass rods would be possible in many engineering, industrial and household applications.

The specified quality standards of metallic bars and rods of tubular and solid section have evolved through improvement of production techniques and in numerous manufacturing and structural applications, and any substitute offered would need to meet certain minimum requirements. The present methods of pressure diecasting metals when applied to the production of zinc alloy rod or bars, result in a gaseous porosity and wide variations in composition and physical properties throughout the cross-section. The rejection rate of rods or bar of zinc alloys produced by known pressure diecasting methods would clearly be such as to be uneconomic.

SUMMARY OF THE INVENTION It is the principal object of the present invention to provide an economic method of pressure die-casting which may be applied to produce zinc based alloy members having physical properties to render them acceptable substitutes formembers produced by other more expensive methods.

With the above stated objects in view there is pro- I vided according to the present invention a method of pressure die-casting a member comprising positioning in an open ended mould cavity having a uniform crosssection a mandrel having at least at one end a portion of a cross-section corresponding to that of the mould cavity, the mandrel being positioned so that such position closes one end of the mould cavity with the remainder of the mandrel extending toward the other end of the mold cavity, delivering molten material under pressure into said one end of the mould cavity, to effect displacement of said mandrel so that said portion of said mandrel moves along said mould cavity toward the other end thereof, and controlling the rate of displacement whereby the cavity formed by the displacement of the mandrel is occupied by the incoming molten material.

The displacement of the mandrel may be effected solely by the force applied thereto by the molten material entering the cavity, or the movement of the mandrel may be assisted by power means. As the material is delivered to the mould under pressure such pressure will be momentarily relieved as the mandrel proceeds along the cavity and will be reinstated when the mandrel comes to rest and may be supplemented by pressure applied to the mandrel, upon completion of the injection cycle. It is practical when the mandrel is assisted by power means to offer resistance to the flow of incoming metal in the die cavity.

The mandrel may be of uniform cross-section to fully occupy the mould cavity when initially placed therein, or may be reduced in cross-section at selected locations to reduce friction during displacement of the mandrel.

The use of the mandrel to initially close the end of the mould cavity and then delivering the molten material against the mandrel, ensures that gases are not trapped in the mould cavity to cause porosity in the resulting casting. This method also avoids the necessity of providing gates and vents in the mould with the resultant formation of spew.

The molten materials may be injected into the mold under a wide range of varying conditions of pressure, temperature and velocity. The degreesof pressure and velocity and conveniently controllable, as desired, by exercising control over the rate of displacement of the mandrel and/or by control of the pressure and velocity factors in the injection apparatus. In the case of some castable materials it may be convenient to maintain a continuous casting operation following the displacement of the initial mandrel and in other cases to make repititious castings by applying pressure periodically to measured quantities of molten materials. In the case of repititious castings, it may conveniently be that the mandrel is withdrawn only partially from the mould and that the tip of the mandrel is used both as a terminating stop for the maintenance of pressure on the molten material during solidification and as a gauge of the length of the cast component.

The range of pressures velocities and temperatures would obviously vary between one castable material and another and, additionally, variations within limited scales would be possible in respect of individual materials. In consequence of this latter aspect the quality of castings produced by the method, the subject of this invention, is influenced by control over such factors as pressures applied in the injection unit, temperatures of the injection unit and the material to be cast, temperatures of the moulds which may contain heating and/or cooling elements but these factors are common to other methods.

In the method, the subject of the present invention however, it is possible to increase the velocity of the injection of the molten material beyond that ordinarily possible. This is due to the fact that as the mandrel within the mould is displaced the speed of injection of molten material is matched to the rate of withdrawal, whereas the rate of injection would normally be restricted by the need to force out through vents gases compressed within the mould, during the injection cycle. Clear and important consequences of this fact are:

a. The mould temperatures need not necessarily be as high as normally, permitting more rapid heat exchange and thus an increased rate of solidification.

b. If so desired and subject to control of molten material and mould temperatures such high velocity injections are practicable as to maintain relativity high liquidity of the injected material until the mould is filled and to maintain optimum static pressures upon the molten material prior to the commencement of solidification.

c. As a consequence of (b) it is also practicable to utilize moulds which upon completion of a primary injection cycle are then subjected to a continued pressure so as to form shapes supplementary to the mandrel shape by means of sliding components within the die itself. For example, a flat bar may be required with a series of bosses or a round rod with a flange. While the replicas would provide the basic shapes the supplementary bosses or flanges can be formed by movable components in the die.

Likewise the basic components could be perforated by similar means.

Another important factor inherent in the method as it relates to high velocity castings is the assistance derived in compacting the casting to an optimum degree by reason of the reactive forces which occur when the mandrel is brought abruptly to rest, or, should it be desired, the capacity, upon completion of the shot, to add to the reactive forces by reversal of the pressure on the mandrel.

A further feature of this process is that the method employed facilitates the rapid casting of relatively long lengths of material which remain liquid until completion of the injection cycle and under continuous pressures, relatively free from gaseous inclusions, until the solidification cycle commences. According to the length and sectional area of such castings it is practical to produce forms of acceptable homogeneity requiring no further processing in order to improve the quality of the product. In some lengths and some sectional areas or for special purposes any interstitial porosity or variation in cyrstalline structure of castings can be readily improved by rapid post-casting processes owing to the greatly improved nature of any castings made according to this method.

In order to exclude gaseous inclusions in the castings produced the injection apparatus and the mould must be mated in such a manner that air or gases are preeluded from entering the mould. There are numerous practical methods of accomplishing this requirement, e.g.:

I. By valves which bleed off air from the injection point until molten material appears prior to the injection mechanism being activated.

II. By using a vertical injection unit in which the molten material can be observed to be at a level precluding air or gases from being injected.

III. By using hooded furnaces to evacuate gases from molten material. It is realized that other arrangements are possible within the scope of the invention defined.

BRIEF DESCRIPTION OF THE DRAWING The drawing depicts one practical arrangement of the moulding equipment for carrying out the present invention.

DETAILED DESCRIPTION OF THE INVENTION The mould 10 comprises a fixed lower portion 11 and a movable upper portion 12, which may be raised and lowered by usual mechanisms, herein conveniently depicted as the power cylinder 14. The lower portion 11 is supported on a suitable bolster 15, and means such as an electric element or gas jets, may be provided to heat the mould. The mould has a cavity 19 extending completely transversely through the mould and is of uniform cross-section throughout its length. The cavity may be circular or any other desired cross-section provided the cross-section is uniform throughout its length. The cavity terminates at one die face in the nozzle recess 21.

The shot chamber 25 is supported on the carriage 26 which is slidably mounted in the guides 27 and 28, to move the shot chamber 25 in a coaxial relationship with the mould cavity 19. The movement of the carriage 26 is effected by the power cylinder 30, which is connected between the lug 32 on the underside of the carriage and the carriage guide 28. As shown in the drawing, the carriage is restricted so that the shot chamber is in position to receive a charge of molten material prior to the commencing of casting. After the shot chamber has been charged, power cylinder 30 is operated to move the carriage 26 towards the mould 10 so that the nozzle 24 of the shot chamber is received by the recess 21 in the mould. The nozzle 24 and recess 21 are arranged to form a seal when brought into engagement, to prevent the escape of molten metal therebetween during delivery of the molten metal from the shot chamber to the mould cavity. The mandrel 20 is connected to the piston rod of the power cylinder 35 which is supported by the fixed mounting 36. The free end of the mandrel 20 cooperates with the bore of the nozzle 24 so as to close the nozzle and prevent the es cape of molten metal during charging of the shot chamber and during movement of the latter so that the nozzle enters the recess 21. The injection cylinder 40 supported on the carriage 26, actuates the plunger 41 of the shot chamber. The shot chamber is charged through the opening 44 which has a surface 45 inclined inwardly and forwardly with respect to the direction of travel of the plunger 41 to bleed air or other gases from the charge during the forward movement of the plunger so that the metal injected into the mould cavity is free of air or other gases.

In operation the apparatus initially occupies a condition as shown in the drawing with the shot chamber retracted from the mould and the mandrel 20 closing the shot chamber nozzle 24. Molten metal is loaded into the shot chamber through the opening 44, until the chamber is completely full of molten metal. The carriage 26 is then moved towards the mould by the power cylinder 30, so that the nozzle 24 sealably engages the recess 21. During this movement of the carriage, the mandrel 20 is retracted by the power cylinder 35, a distance and at a rate so that it remains in position to seal the nozzle 24. The injection cylinder 40 is then operated to cause the plunger 41 to move forward in the shot chamber and the initial movement of the plunger displaces molten metal back through the opening 44. This initial displacement of molten metal through the opening 44 ensures that there is no air or gas trapped in the molten metal and the inclined face 45 provides a progressive closing of the opening as the plunger 41 moves towards the nozzle so that there is not an abrupt cut-off of the passage for the escape of gases. Once the forward end of the plunger 41 passes the lower edge of the surface 45, the metal in advance of the plunger becomes subjected to the full injection pressure and causes withdrawal of the mandrel 20 so that injection of molten metal into the cavity commences. The rate of retraction of the mandrel 20 in relation to the rate of injection of the molten metal is controlled by the resistance offered in the cylinder 35 so that the desired velocity of the incoming metal is attainable.

After the mandrel has been retracted to the desired position and the mould cavity filled with metal from the shot chamber, the mould is opened and the cast rod removed from the mould. The apparatus is then re turned to the condition shown in the drawing for the next casting operation.

I claim:

1. A method of pressure die-casting a member comprising positioning in an open ended mould cavity of uniform cross-section a mandrel having at least at one end a portion which has a cross-section corresponding to that of said mould cavity, said mandrel being positioned so that said portion closes one end of said mould cavity with the remainder of the mandrel extending toward the other end of said mould cavity, delivering molten die-casting material under pressure to said one end of said mould cavity, to effect displacement of said mandrel so that said portion of said mandrel moves along said mould cavity toward the other end thereof, and controlling the rate of displacement whereby the cavity formed by the displacement of the mandrel is occupied by the incoming molten material.

2. A method as claimed in claim 1 wherein the displacement is effected against a force opposing movement of the mandrel towards said other end of the cavity.

3. A method as claimed in claim 1 wherein the shape of the die cavity is varied after completion of the delivery of the molten material and before solidification of the material so that the shape of the cast member differs from the shape of the mandrel.

4. A method of pressure die-casting a member comprising positioning in an open ended mould cavity of uniform cross-section a mandrel having at least at one end a portion of a cross-section corresponding to that of said mould cavity, said mandrel being positioned so that said portion closes one end of said mould cavity with the remainder of said mandrel extending toward the other end of said mould cavity, delivering molten die-casting material under pressure to said one end of said mould cavity, to effect displacement of said mandrel so that said portion of said mandrel moves along said mould cavity toward the other end thereof, and controlling the rate of displacement whereby the cavity formed by the displacement of the mandrel is occupied by the incoming molten material, and applying a force to the mandrel upon completion of the injection of the desired quantity of molten material to subject the molten material to pressure during solidification. 

1. A method of pressure die-casting a member comprising positioning in an open ended mould cavity of uniform crosssection a mandrel having at least at one end a portion which has a cross-section corresponding to that of said mould cavity, said mandrel being positioned so that said portion closes one end of said mould cavity with the remainder of the mandrel extending toward the other end of said mould cavity, delivering molten diecasting material under pressure to said one end of said mould cavity, to effect displacement of said mandrel so that said portion of said mandrel moves along said mould cavity toward the other end thereof, and controlling the rate of displacement whereby the cavity formed by the displacement of the mandrel is occupied by the incoming molten material.
 2. A method as claimed in claim 1 wherein the displacement is effected against a force opposing movement of the mandrel towards said other end of the cavity.
 3. A method as claimed in claim 1 wherein the shape of the die cavity is varied after completion of the delivery of the molten material and before solidification of the material so that the shape of the cast member differs from the shape of the mandrel.
 4. A method of pressure die-casting a member comprising positioning in an open ended mould cavity of uniform cross-section a mandrel having at least at one end a portion of a cross-section corresponding to that of said mould cavity, said mandrel being positioned so that said portion closes one end of said mould cavity with the remainder of said mandrel extending toward the other end of said mould cavity, delivering molten die-casting material under pressure to said one end of said mould cavity, to effect displacement of said mandrel so that said portion of said mandrel moves along said mould cavity toward the other end thereof, and controlling the rate of displacement whereby the cavity formed by the displacement of the mandrel is occupied by the incoming molten material, and applying a force to the mandrel upon completion of the injection of the desired quantity of molten material to subject the molten material to pressure during solidification. 